Recently, as an electrochemical element, for example, a lithium ion secondary battery as a typical example of a non-aqueous electrolyte secondary battery has been widely used because it has a high electromotive force and a high energy density while it is light. Demands for lithium ion secondary batteries are increasing for a driving power supply of various kinds of portable electronic equipment such as a cellular phone, a digital camera, a video camera, and a notebook-sized personal computer, as well as mobile telecommunication equipment.
A lithium ion secondary battery includes a positive electrode including a lithium-containing composite oxide; a negative electrode including a negative electrode active material absorbing and releasing lithium metals, lithium alloys or lithium ions; and an electrolyte.
Recently, instead of carbon materials such as graphite that have been conventionally used as a negative electrode material, elements having a property of absorbing lithium ions and having a theoretical capacity density of more than 833 mAh/cm3 have been studied and reported. An example of the elements for a negative electrode active material, which have a theoretical capacity density of more than 833 mAh/cm3, includes silicon (Si), tin (Sn), germanium (Ge) to be alloyed with lithium, and oxide and alloys thereof. Among them, since silicon-containing particles such as Si particles and silicon oxide particles are cheap, they have been widely studied.
However, these elements increase their volume when they absorb lithium ions at the time of charging. For example, when the negative electrode active material is Si and Si is changed to Li4.4Si, the volume is increased by 4.12 times as the volume at the time of discharging, where Li4.4Si denotes the state in which the maximum amount of lithium ions is absorbed.
Therefore, in particular, when a thin film of the above-mentioned elements is deposited on a current collector by a CVD method, a sputtering method, or the like, so as to form a negative electrode active material, a negative electrode active material expands/contracts by absorption/release of lithium ions. Exfoliation of a negative electrode active material from a negative electrode current collector may occur because of deterioration in adhesion while charge and discharge cycles are repeated.
In order to solve the above-mentioned problems, a method of providing convex and concave portions on the surface of a current collector, depositing a negative electrode active material thin film thereon, and forming gaps in the thickness direction by etching is disclosed (see, for example, patent document 1). Similarly, a method of providing convex and concave portions on the surface of a current collector, forming resist patterns so that convex portions become opening, electro-depositing a negative electrode active material thereon, and then removing the resist, thus forming a columnar body (see, for example, patent document 2). Furthermore, a method of disposing a mesh above the current collector, and depositing a negative electrode active material thin film through the mesh, thereby suppressing the deposition of the negative electrode active material in a region corresponding to the frame of the mesh is proposed (see, for example, patent document 3).
Furthermore, a method of providing convex and concave portions on a surface of a current collector and forming a film-like negative electrode material thereon in a way in which the negative electrode material is inclined with respect to the surface perpendicular to a main surface of the negative electrode material is disclosed (see, for example, patent document 4). This shows that stress generated by expansion and contraction of charge and discharge can be distributed into the parallel direction and the vertical direction to the surface of the negative electrode material, thereby suppressing the generation of wrinkles and exfoliation.
In secondary batteries shown in patent documents 1 to 3, a thin film of a negative electrode active material is formed in a columnar shape and gaps are provided between the columnar shapes, thus preventing exfoliation and wrinkles from occurring. However, since the negative electrode active material contracts at the time when charge is started, a metal surface of the current collector may be exposed through gaps. Thereby, since the exposed current collector faces the positive electrode at the time of charging, lithium metal tends to be precipitated, which may reduce safety and capacity. Furthermore, when the height of the columnar negative electrode active material is increased or the gap interval is reduced in order to increase battery capacity, in particular, since the tip (opening end) of the columnar negative electrode active material is not regulated by the current collector and the like, it expands more as compared with the vicinity of the current collector with the proceeding of charging. As a result, columnar negative electrode active materials are brought into contact with each other and pushed to each other in the vicinity of the tip. Thereby, the exfoliation between the current collector and the negative electrode active material and wrinkle of the current collector may occur. On the other hand, when the interval between gaps is increased in order to avoid contact between negative electrode active materials at the time of expansion, lithium metal tends to be precipitated. In other words, in the above-mentioned secondary batteries, exfoliation between the current collector and the negative electrode active material and wrinkles in the current collector cannot be prevented. Furthermore, since an electrolytic solution is trapped in the gaps in the columnar-shaped negative electrode active materials that have been expanded and brought into contact with each other, movement of lithium ions when discharge is started is prevented. As a result, in particular, there have been problems in characteristic of discharge at a high rate (hereinafter, referred to as “high-rate discharge”) or discharge at a low temperature environment.
Furthermore, in a structure of a secondary battery disclosed in patent document 4, as shown in FIG. 11A, with negative electrode active material 53 formed by inclining (at an angle θ), current collector 51 can be prevented from being exposed. Then, precipitation of lithium metal can be prevented in advance. However, similar to patent documents 1-3, as shown in FIG. 11B, negative electrode active material 53 larger expands as compared with the vicinity of current collector 51 with the proceeding of charging. Consequently, columnar negative electrode active materials are brought into contact with each other in the vicinity of the tips and pushed to each other as shown by arrows in FIG. 11B. As a result, exfoliation between current collector 51 and negative electrode active material 53 and wrinkles in current collector 51 may occur. Furthermore, stress by expansion and contraction on the negative electrode active material concentrates on a connection interface between the negative electrode active material and the convex portion. As a result, as the charge and discharge cycle proceeds, a negative electrode active material may be peeled off from the connection interface on the convex portion due to the stress, thus reducing the reliability. Furthermore, since an electrolytic solution is trapped in gaps 55 between columnar shaped negative electrode active materials that have been expanded and brought into contact with each other, movement of lithium ions is prevented when discharge is started. In particular, there has been a problem in discharge characteristic of high-rate discharge, discharge in a low-temperature environment, and the like.    [Patent document 1] Japanese Patent Application Unexamined Publication No. 2003-17040    [Patent document 2] Japanese Patent Application Unexamined Publication No. 2004-127561    [Patent document 3] Japanese Patent Application Unexamined Publication No. 2002-279974    [Patent document 4] Japanese Patent Application Unexamined Publication No. 2005-196970